Modem

The modem ( formed from modulator and demodulator acronym or portmanteau word, hence the modem) is used to exchange digital signals over long transmission paths between two digital devices. The transmitting modem, a digital signal is modulated onto a carrier frequency in the high frequency range, the receiving modem is recovered from the original information by demodulating. This modem of newer standards DSL operate at higher carrier frequencies and larger bandwidths on the telephone line as the models after the traditional standard ( " narrow band ").

• 3.2.1 Leased Line Modem
• 3.2.2 DSL modem
• 3.2.3 Cable Modem
• 3.2.4 Power Line Modem

• 3.3.1 Wireless Modem
• 3.3.2 Satmodem

History of the modem in Germany

At the German Federal Post Office modulation methods were used early in the broadcast engineering and later in the carrier frequency technology (wireless modulation). The use in the network-based communication ( wireline modulation) began in Germany in 1966 with the introduction of the first modems by the German Federal Post Office.

These devices had a sheet metal housing with external dimensions of 60 cm × 30 cm × 20 cm and were designed for wall mounting. They were called transition Modem D 1200 S. The number in 1200 was for the maximum transmission speed in bit / s, the letter S for " Serial Transmission " ( to distinguish it from modems with parallel transfer). As a direct successor, there were modems for desktop use with the designation unit post Modem D 1200 S and Unity Post Modem D 200 S. These correspond to the ITU- T Recommendation V.23 ( 1200 bit / s half-duplex operation, possibilities of an auxiliary channel and a synchronous transmission by use of a clock module) or V.21 ( asynchronous duplex transmission with 200 or 300 bit / s).

Until the mid- eighties it was not allowed in Germany, as in many other countries, to connect other than post your own modem to the telephone line. The modem counted as network termination, which also belonged to the telephone network and thus to the jurisdiction of the state Federal Post Office as the lead. As a still increasing number of private modems were used because they were significantly cheaper, faster and more user friendly than the post- modem, the modem monopoly could no longer maintained and has been removed.

The German Federal Post Office also requested that those connected to their modem devices had an approval of the Telecommunications Technology Central Office. Since it was difficult to achieve especially with larger computer systems, some manufacturers went on to build so-called modem adapter. These devices were up to some LEDs to display the modem control signals virtually functionless, but easily got the approval (see picture on the right with the MODAP the DEC under the DS 200 S).

Modulation techniques

• Phone: ITU- T standards (until 1992 CCITT)
• Cable: half-duplex ( one pair ), Duplex ( two pairs )

Modem variants

Dial-up modem

Telephone modem

Telephone modems for POTS connections are adapted to the specificities of the telephone network. Is solely responsible for the transmission of the frequency band from 300 Hz to 3400 Hz. Connections to a digital exchange to extend this band to 0 to 4000 Hz

The first telephone modems for computer mass market were not electrically coupled to the telephone network, but made ​​the link by means of appropriate transducers - the so-called acoustic - the receiver of an existing phone manufactures. This was mechanically attached to the modem. Noise quickly led to transmission errors, which is why the shells of the acoustic coupler were often wrapped in pillows. It was baud with 110, later 300 baud with the modulation method Rated transferred. Since the data to be transmitted were typically coded according to the specifications of the serial data transmission lines ( RS-232 or V.24 ), it was found, in practice, a lower effective data rate. These early types of modem had to be subjected to the approval of a type pattern examination of the German Post Office.

The inequality " data rate is less than baud rate " had to be reset by the realization of intelligent modems with built-in data compression. During this time implemented by the telecommunications company Hayes also the current de facto standard for modems, called the AT command set, which was later acquired and expanded by numerous manufacturers. More efficient modulation scheme such as PSK and QAM and an intelligent measurement and negotiation of the maximum for the management and the remote possible baud rate for the connection further increased the achievable data rate.

Due to the ever increasing performance of modem processors today, there are other features, but beyond the basic framework of the modem concept. Mention may be made in particular the fax function ( fax modem ) and the answering machine function ( voice modem ). There are even models that allow the computer- supported telephony. The support is reflected by additional commands as part of the AT command set. In addition modems with integrated fax and e- mail protocols ( POP3/SMTP ), and SMS functionality exist for autonomous shipping and receiving of messages.

In the analog telephone network, where the transmission bandwidth is limited to 3.1 kHz, according to the Shannon theorem, the maximum data transfer rate is at normal line quality to 30 to 40 kbit / s limited. A download data rate of 56 kbit / s ( V.90, V.92 ) is in the telephone network only with an analog connection possible, which is coupled to a digital exchange. In this case the modem is synchronized with the clock of the exchange converter. The sending peer, such as a dial-in node, but it has to be fully digital. However, the upload speed remains limited analog.

Modem connections via the telephone system are often referred to as a dial -up connection as a dialing is necessary before the establishment of the data connection. Examples of Modemwählverbindungen are eg BTX, Datex -P or the analog dial into the Internet through an Internet provider. The dial-in via ISDN differs from it in so far as there everything goes digital, so no longer is modulated and thus no more modem is involved. Nevertheless, one often reads the misnomer ISDN modem for an ISDN line device or an ISDN card.

While were delegated in the early phase mainly Telegraph tasks to modem, it came later to the development of the mailbox scene that worked with protocols such as Kermit or Z-Modem. In addition, found himself in the commercial sector as X.25 network layer. Today at the network layer mainly dominated IP for data exchange via telephone modem.

Fax modem

Fax modems dominate in addition to the usual data transfer feature is also a protocol for transmission of faxes. Most fax modem can transmit faxes at 14400 bit / s. The transmission takes place over ordinary telephone lines.

Using a fax modem, you can get a computer that is used as a fax machine - often only to send, but also receiving is possible.

Soft Modem / winmodem

Soft modems are special modems, in which some parts of the hardware functionality have been outsourced for cost reasons in the device driver. Often the sound card or the corresponding module of the motherboard takes over the actual modem functions; physically only the contacts to the telephone network are available. Winmodems are soft modems that have been especially made ​​for Microsoft Windows. They usually work only with the time of their manufacture recent versions of Windows. On alternative operating systems such as Linux, they can not be used or only with difficulty, since usually there are no drivers.

Leased Line Modem

Leased Line Modem

Unlike dial-up modems, leased line modems provide a point-to - point connection. So you are always firmly connected to the same remote site; a dialing before the connection is not necessary. Leased lines are usually used by banks and large corporations. Comparable cost-effective solutions for the home are usually only quasi- dedicated lines, because the provider often provides for a forced separation of the compound in the daily cycle.

While the bandwidth in the analog telephone network is limited due to technical and economic considerations, allowing normal - but partly also shielded - two-wire telephone lines, in this case quite high bandwidth.

DSL modem

DSL modems allow much higher transmission rates. In order to transfer the conventional telephone signal and the additional high-speed signal on the same cable at the same time, the high-speed signal is modulated up to a higher frequency range, before both signals are then placed on the same line. On the receiver side, the signals ( a diplexer ) again separated by a so-called splitter, and fed to the high-speed signal of the demodulation in the local modem.

Common data rates for ADSL reach 25 Mbit / s download. The upload is limited ADSL to a lower value. The data rate is subject at large distance from the central office for technical reasons limitations. In the exchange is usually coupled to a purely digital network backbone.

Cable Modem

Cable modems for data transmission via cable TV networks are seen as a type of leased line modem. Once a telephone modem is used partly in addition, to allow a return channel. This technique is no longer necessary because cable providers such as Germany, Kabel BW and Unity Media to expand its cable network now with return channel. About TV cable modems today the same data rate as in the data transmission using DSL modems are offered, such as data rate 150 Mbit / s even go beyond that. The cable operators will usually also implement telephony via cable modems.

Power line modem

And the modulation of data signals to power lines is possible. A special, but simple application is the so-called ripple control technology of the power company, with which, for example, switching the electric meter between day and night electricity is accomplished.

More recently, also marketing attempts were for high -speed modems (usually to about 1 Mbit / s) under the collective term PLC ( Power Line Communication ) undertaken, but on trial phase never were on their way and in the end, not at the price-performance ratio and the transmission reliability of the DSL could come close technique. Also produced by this technique RF interference are not without problems. The technique typically makes use of a number of individual carrier frequencies in the range between 500 kHz and 10 MHz to the modulation and demodulation of user data.

Radio modem

Radio modem

Often radio modems are in other devices - integrated, and the corresponding channel is used several times (eg Tonrufsystemen for voice and data) - such as mobile phones.

In the radio sector, there are numerous applications that allow remote control or remote control objectives are achieved by modulation. An example are radio remote controls in model.

The GSM, UMTS and LTE network use for data transmission via radio modulation techniques with digital modulation, while the previous system C network still used analog modulation. Here is the digital useful signal is converted by the modulator into a radio signal. This makes the transmission much less sensitive to disturbances, since the digital useful signal in contrast to the continuous analog signal can take known, discrete values ​​only, to which the receiver (not too strong) interference in the transmission can still close even with.

The increasingly popular UMTS modems are also radio modems.

Satmodem

Satmodems or satellite modems are used for data transmission via a communication satellite. This converted satellite modems an input bit stream into a radio signal and vice versa. There is a wide range of satellite modems from cheap devices to access the Internet at home to expensive multifunctional devices for enterprise use. Here are devices that have only one demodulator for the downloading of data via satellite, also known as satellite modems. In this case, the data uploaded by a conventional telephone modem, a DSL modem or ISDN are transferred.

Modem designs

The typical PC modem is an external device with a flat design with a serial interface. It is usually connected via RS -232 or USB increasingly to a computer. The power supply is normally done by an AC adapter. For the status display often located at the front of a row of LEDs that indicate the status of the interface lines.

In professional fields, there will be parallel a design that allows installation in 19 -inch casing. In the industrial sector, a housing for DIN rail mounting has established itself in the cabinet for modems.

An alternative design for modems is the plug-in card form for a standardized slot (previous PCI in a stationary PCs, Mini - PCI or PCMCIA for laptops ) or a proprietary socket. Here is usually an additional enclosure or shielding in place to prevent interference of PC and telephone line. The status display of such a modem is mostly implemented by computer software on the computer screen.

Modems may also be integrated into the motherboard of a computer. This is, however, recommended only limited, since it is not unlikely depending on the region that a modem will be damaged by power surges on the phone line. As a result, can then damage the entire computer. In contrast, modular systems allow a much more differentiated and therefore more cost-effective error handling. To minimize the likelihood of damage to the machine, there are also power strips with an overvoltage protection for the telephone line.

Manufacturers of modem hardware

• AVM GmbH
• Bausch Datacom, 1990 Manufacturer of the first low-cost modems for Vobis
• Baycom, manufacturer of amateur radio modems
• Cisco Systems
• Conexant (formerly Rockwell) provides, in particular OEM chipsets
• ConiuGo manufacturer of industrial wireless modem ( since 1998)
• Devolo (formerly ELSA AG)
• Diamond Multimedia, with the brand and models such as Supra Supra Express
• Dr. Neuhaus, with the modems Smarty and FURY series, in addition to ELSA one of the first manufacturers of modems with PTT approval for the private (since 1988)
• FlexDSL Telecommunications AG, manufacturer of industrial modems
• Hayes, put the AT standard
• IBM
• Keymile
• Miro
• Motorola
• Phoenix Contact, manufacturer of industrial modems
• RAD Data Communications, a producer of industrial modems
• Siemens, manufacturer of industrial modems
• Sprint Nextel, U.S. telephone company
• Telebit whose " Trail Blazer" had a similar modulation methods such as DSL today
• Tixi.Com, manufacturer of industrial modems
• U.S. Robotics/3Com, with the brand Courier
• Westermo, manufacturer of industrial modems
• ZyXEL